Increasing ammonia emissions (primarily from agriculture) contribute to PM formation, eutrophication and biodiversity loss. Uncertainties in ammonia emissions thus pose challenges to the development of environmental control strategies and understanding the global transport pathways of reactive nitrogen. Top-down estimates have historically provided an important constraint on US ammonia emissions inventories. This talk will present recent evaluations of ammonia sources using multiple approaches. Constraints on ammonia are first derived using remote sensing observations from the TES instrument aboard the Aura satellite with the GEOS-Chem adjoint-based inverse model. We next consider ammonia sources inferred using the same modeling tool, yet constrained with observations of wet deposited ammonia and ammonium. In addition, we consider the degree to which biases in primary source estimates, versus treatment of ammonia bidirectional fluxes, may help explain differences between the initial and optimized ammonia distributions. The impacts of updating the treatment of ammonia in air quality models on estimated efficiency of aerosol mitigation strategies are assessed under present day conditions, as well as conditions from future scenarios that predict rising ammonia abundances.